[clang] [NFC][clang] Split clang/lib/CodeGen/CGBuiltin.cpp into target-specific files (PR #132252)

[Nikita Popov via cfe-commits]

nikic wrote:

I looked into why the tiny files are slow to compile, and it seems to mostly come down to CodeGenFunction.h being expensive (including it adds ~3s to the build time). I submitted https://github.com/llvm/llvm-project/pull/134100 to improve this a bit.

https://github.com/llvm/llvm-project/pull/132252